<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #result {
      width: 200px;
      height: 100px;
      border: 1px solid #90b
    }
  </style>
</head>
<body>
  <button id="btn">点击发送请求</button>
  <div id="result"></div>
  


  <script>

    const result = document.getElementById('result')
    const btn = document.getElementById('btn')
    
    btn.onclick = function () {

      const xhr = new XMLHttpRequest()

      //超时设置
      xhr.timeout = 2000
      //超时回调
      xhr.ontimeout = function () {
        alert('超时,请稍后重试')
      }

      // 网络异常回调
      xhr.onerror = function () {
        alert('网络异常,请稍后重试')
      }

      xhr.open('GET', 'http://127.0.0.1:8000/delay') 

      xhr.send()

      xhr.onreadystatechange = function () {
        if(xhr.readyState === 4) {
          if(xhr.status >=200 && xhr.status<300) {
            result.innerHTML = xhr.response
          }
        }
      }
    }






  </script>







</body>
</html>